| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 97 === In this study, selected from Al, Co, Cr, Fe, Ni, and Ti, various quinary equal- and non-equal-mole and senary high-entropy alloys (HEAs) are made by vacuum-arc melting. The as-cast alloys are sequentially homogenized at 1100℃ for 24 h and water-quenched to obtain as-homogenized alloys. Microstructure examination, coefficient of thermal expansion (CTE) measurement, and Hall effect up to 9T are performed. Microstructure is composed of FCC, BCC, full Heusler, and sigma phases. Most alloys are of magnetic. CTE of as-homogenized alloys drops at Curie point due to magnetic order-disorder transition, the so-called magnetic Invar effect. Curie points thus obtained are in the intervals of 480 K to 750 K and 780 K to 870 K for equal- and non-equal-mole as-homogenized alloys, respectively.
Anomalous Hall effect occurs in most alloys, and electron-like and hole-like electrical conduction behaviors emerge in 50 %-50 % ratio of alloys. Similar to their pure elements and conventional alloy counterparts, carrier concentration of HEAs in this study at 5 K and 300 K varies from 10^22 to 10^23 cm^-3, while lower than their pure elements and conventional alloy counterparts, mobility of carriers varies from 0.21 to 2.61 cm^2 V^-1 s^-1, due to multielement mixing-caused heavy imperfection in alloy lattices, making strong electron scattering effect and impeding the electron transport. The effective number of free electrons in the free electron model is the valence number, ranging from 0.171 to 4.716. The residual electrical resistivity ratio (RRR) for equal-mole and non-equal-mole alloys ranges from 0.988 to 1.259, and from 1.114 to 1.174, respectively. The sign of Hall coefficient is positive for RRR intervals of 1.032 to 1.056 and 1.139 to 1.207 for equal-mole alloys, while it is positive for RRR in the interval of 1.158 to 1.173 for non-equal-mole AlxCoCrFeNi alloys. Otherwise, it has negative sign. The sign of Hal coefficient is explained t have something to do with the interaction between phonon and lattice defects that determines the shape of temperature-dependent Brillouin zone and the Fermi surface.
HEAs have simple crystal structure, thus they obey the Bloch theorem. One can apply a modified Kronig-Penney model t them with an idea of “average” effect in potential.
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